The present invention relates to video signal reproducing apparatus and methods for time base compensation of reproduced luminance and color signals.
Existing video tape recorders record and reproduce FM modulated luminance signals and low frequency converted color signals with the use of a rotary magnetic recording/reproducing head. The recording/reproducing head includes a tape guide drum operative to rotate at a frame frequency and a pair of magnetic heads affixed thereto such that they alternately contact a magnetic tape wound obliquely about the circumference of the tape guide drum to effect recording or reproduction of the luminance and color signals.
The existing video tape recorders employ either a phase inversion system or a phase shift system for respectively inverting or shifting the phase of the color signal carrier between adjacent recording tracks to facilitate crosstalk elimination upon signal reproduction. In the reproduction mod,, such crosstalk elimination is accomplished by combining the reproduced low frequency converted color signal with a previously reproduced portion thereof delayed by one horizontal line. Consequently, a delay of one horizontal period is introduced in the reproduced signal which is then compensated by correspondingly delaying the reproduced luminance signal by one horizontal line.
The reproduced luminance and color signals are also subjected to filtering which introduces delays in addition to those created by the crosstalk elimination process. It is, therefore, necessary to adjust the delay times produced by filtering so that a further time difference does not occur between the luminance and color signals. It will be appreciated that the foregoing measures undesirably increase the cost of existing video tape recorders as they require a delay line to compensate for the delay introduced by crosstalk elimination, and by requiring careful adjustment of filtering circuitry so that a time differential is not introduced between the luminance and color signals thereby.
In the reproduction mode of the existing video tape recorder, the level of the reproduced signal is continuously monitored so that when it falls below a predetermined value, it is determined that a dropout has occurred. Upon such determination, a dropout detection signal is produced for use in subsequent processing to replace the portion of the signal in which the dropout has occurred with a vertically corresponding portion of a preceding line of the signal which has been subjected to a one horizontal period delay for this purpose.
Digital time base compensation (TBC) is another signal processing technique carried out in the reproduction mode for the purpose of eliminating time base fluctuation, also referred to as "jitter", in the reproduced video signal. Such time base compensation is carried out by writing the reproduced luminance signal in a memory with the use of a clock signal synchronized with the luminance signal and subsequently reading the stored luminance signal from memory with the use of a stable read clock. A further technique employed in the reproduction mode serves to eliminate noise in the reproduced color signal with the use of a cyclic comb filter which requires vertical correlation of the color signal for effective operation. A similar noise reduction capability is provided for the luminance signal, together with correlation detection for generating a detection signal indicating the presence or absence of vertical correlation in the reproduced luminance signal. Accordingly, in order to avoid deterioration of the color signal due to a lack of correlation thereof, the noise elimination process for the color signal is selectively disabled at such times that the detection signal indicates the absence of vertical correlation.
The reproduced luminance signal and low frequency converted color signal typically possess different time base errors. Therefore, it is preferable to carry out noise elimination after time base compensation to prevent the occurrence of a time shift between the color signal and the correlation detection signal. However, when dropout compensation is carried out after time base compensation, an undesirable time shift is introduced between the time base compensated luminance signal and the dropout detection signal.
It will be appreciated that the magnitude of time base fluctuation which can be corrected by a given time base compensator depends on its memory capacity. That is, when the time base fluctuation becomes too great, the write address outruns the read address of the time base compensator memory, such that certain portions of the video signal are consequently lost. When this occurs, it is typically necessary to re-read a picture previously stored in memory. It is, therefore, preferable to avoid such outrunning and in one known technique for doing so, a control signal is produced by frequency division of the read clock which in turn is used by the drum servo circuitry for controlling the phase of the drum's rotation in order to maintain the average values of the write and read clocks essentially equal.
While the foregoing technique is effective for minimizing outrunning in the time base compensation memories, it will be appreciated that the phase of the drum will not be synchronized with the read clock when the speed of the magnetic tape is varied, for example, in a slow reproduction mode, a cue reproduction mode or a jog mode in which continuous speed control over a range of values is employed for editing purposes. Accordingly, in such non-standard speed modes of operation, the time base compensation circuitry typically is disabled and the read clock is uncoupled from the drum servo system. When the standard playback mode is recommenced and time base compensation then reactivated, the read clock signal is again provided to the drum servo system for stabilizing the write and read clocks. However, when this occurs the phase of the servo reference signal varies discontinuously since the phase of the drum's rotation has been decoupled from the control signal produced by dividing the read clock, and as a consequence, it takes a relatively long time for the drum servo system to lock with the control signal. Therefore, an undesirably long delay is caused between the recommencement of the normal playback mode and the operative state of the time base compensator.